This is a continuing effort to study the development of the nervous system of the rat from the early embryonic period (closure of the neural tube) into early adulthood. The investigation includes the peripheral nervous system, spinal cord and the brain and the aspects studied are categorized under neurogenesis (time and site of origin of neurons), morphogenesis (migration and settling pattern of neurons), axonogenesis, dendrogenesis and synaptogenesis. Present efforts concern the correlation of the embryonic development of the medulla and brain stem (morphogenesis, axonogenesis and dendrogenesis) with the recently determined time of origin of neurons in the major components of this region.